Polymeric electro-optic (EO) materials have demonstrated enormous potential for core application in a broad range of next-generation systems and devices, including phased array radar, satellite and fiber telecommunications, cable television (CATV), optical gyroscopes for application in aerial and missile guidance, electronic counter measure (ECM) systems, backplane interconnects for high-speed computation, ultraquick analog-to-digital conversion, land mine detection, radio frequency photonics, spatial light modulation and all-optical (light-switching-light) signal processing.
Nonlinear optic (NLO) materials are capable of varying their first-, second-, third- and/or higher-order polarizabilities in the presence of an externally applied electric field or incident light (two-photon absorption). In many current telecommunication applications, the second-order polarizability (hyperpolarizability or β) is of great interest. The hyperpolarizability is related to the change of a NLO material's refractive index in response to application of an electric field. A more complete discussion of nonlinear optical materials may be found in D. S. Chemla and J. Zyss, Nonlinear optical properties of organic molecules and crystals, Academic Press, 1987 and K.-S. Lee, et al. Polymers for Photonics Applications I, Springer (2002)
Many NLO molecules (chromophores) have been synthesized that exhibit extremely high molecular electro-optic properties. The product of the molecular dipole moment (μ) and hyperpolarizability (β) is often used as a measure of molecular electro-optic performance due to the dipole's involvement in material processing. One chromophore originally evaluated for its extraordinary NLO properties by Bell Labs in the 1960s, Disperse Red (DR), exhibits an electro-optic coefficient μβ˜580×10−48 esu. Current molecular designs, including FTC, CLD and GLD, exhibit μβ values in excess of 10,000×10−48 esu. See Dalton et al., “New Class of High Hyperpolarizability Organic Chromophores and Process for Synthesizing the Same”, WO 00/09613.
Nevertheless extreme difficulties have been encountered translating microscopic molecular hyperpolarizabilities (β) into macroscopic material hyperpolarizabilities (χ(2). Molecular subcomponents (chromophores) must be integrated into NLO materials that exhibit (i) a high degree of macroscopic nonlinearity and (ii) sufficient temporal, thermal, chemical and photochemical stability. Simultaneous solution of these dual issues is regarded as the final impediment in the broad commercialization of EO polymers in numerous government and commercial devices and systems.
The production of high material hyperpolarizabilities (χ(2)) is limited by the poor social character of NLO chromophores. Commercially viable materials must incorporate chromophores at large molecular densities with the requisite molecular moment statistically oriented along a single material axis. In order achieve such an organization, the charge transfer (dipole) character of NLO chromophores is commonly exploited through the application of an external electric field during material processing that creates a localized lower-energy condition favoring noncentrosymmetric order. Unfortunately, at even moderate chromophore densities, molecules form multi-molecular dipolarly-bound (centrosymmetric) aggregates that cannot be dismantled via realistic field energies. To overcome this difficulty, integration of anti-social dipolar chromophores into a cooperative material architecture is commonly achieved through the construction of physical barriers that limit proximal intermolecular relations. This has been successfully accomplished through (i) surrounding individual molecules with sterically hindering constituents or (ii) covalently binding molecules to secondary organizing superstructures such as on polymeric backbones or within dendrimeric formations. Other methods, such as self-assembling superlattices, have been proposed by Tobin Marks and others but are unlikely to produce near-term macroscopically-useful results. See K.-S. Lee, et al. (2002); Keinan S. et al., Chem. Mater., 16, 1848-1854 (2004); Koeckelberghs, G. et al., Marcromolecules, 36, 9736-9741 (2003); Robinson, B. H. et al. J. Phys. Chem. A, 104, 4785-4795 (2000); L. Dalton et al., “The Role of London Forces in Defining Noncentrosymmetric Order of High Dipole Moment-High Hyperpolarizability Chromophores in Electrically Poled Polymeric Films”, Proceedings of the National Academy of Sciences USA, Vol. 94, pp. 4842-4847 (1997).
Nevertheless, the most daunting problem in the production of commercially successful NLO polymers is the issue of resultant long-term material stability. Although molecular organization techniques have produced extremely high-performance materials (exhibiting sub-1-volt drive voltages and switching frequencies in excess of 100 Gb/s), the manufacture of a commercial quality high-stability polymer-based devices operating at even 10 Gb/s is only now on the verge of reality. See, L. Dalton et al., “Synthesis and Processing of Improved Organic Second-Order Nonlinear Optical Materials for Applications in Photonics”, Chemistry of Materials, Vol. 7, No. 6, pp. 1060-1081 (1995); and Shi Y. et al., Science, 288, 119-121. This failing is primarily due to the reinstitution of centrosymmetry as a result of molecular mobility over time. Three solutions have been envisioned to resolve this issue: (i) incorporation of chromophores in high glass transition (Tg) host polymers; (ii) backbone and dendrimeric single-point polymer integration; and (iii) multi-point crosslinked integration. The use of high Tg polymers has yet to show satisfactory results due to thermal-induced nucleophilic degradation of NLO chromophores. Single-point integration techniques wherein the chromophore is attached to a polymeric superstructure via one point on the chromophore (usually on the electron donating amine) have similarly demonstrated insufficient thermal character presumably due to the residual latitude of molecular mobility; in addition to thermal randomization, mobility is partially induced over operation lifetime by motion as a result of photo-stimulated cis-trans isomerization. Multi-point and double-ended crosslinked (DEC) integration strategies are the only techniques that have demonstrated the ability to meet thermal requirements. See Kajzar, F. et al. Organic Thin Films for Waveguding Nonlinear Optics, Gordon (1996).
Thus, the effectiveness of organic NLO materials having high hyperpolarizabilities is limited by the tendency of these materials to aggregate when processed as well as the thermal stability of those resultant materials. Accordingly, there exists a need for improved nonlinear optically active materials having large hyperpolarizabilities and that when employed in electro-optic devices, exhibit large electro-optic coefficients and high thermal stability. The present invention seeks to fulfill these needs and provides further related advantages by introducing spacer systems that separate individual chromophores thereby preventing aggregation and providing for multi-point material integration for long-term thermal stability.